High Efficiency Reverse Osmosis (HERO™)
Technology for Difficult to Treat Water
High Efficiency Reverse Osmosis (HERO) is a reverse osmosis technology specially designed to treat high silica and difficult to treat wastewater. By effectively preventing membrane fouling or scaling, the high efficiency reverse osmosis process offers the end user higher recoveries as well as significant operational benefits. The process has an excellent track record of treatment of cooling tower blowdown where conventional reverse osmosis (RO) is generally considered to be a less effective technology.
In some instances, HERO™ may be used to replace or supplement an evaporation process as a cost-effective method of achieving zero liquid discharge (ZLD). Aquatech has unmatched experience with HERO™ technology with over 60 installations around the world.
Basic process consists of three steps: Hardness and suspended solids removal, Carbon dioxide removal, and reverse osmosis treatment at elevated pH.
HERO™, a Reverse Osmosis (RO) based process, is specially designed to treat aggressive feed waters. Constituents which often pose problems to conventional RO processes are handled with ease by HERO™. Examples of these constituents are biological matter, oil and grease, and silica. By effectively preventing these constituents from being a root cause to membrane scaling, the HERO™ process offers the operator and endusers HIGHER RECOVERIES AND OPERATIONAL BENEFITS. Both of these lead to REDUCED COSTS through:
• Less Water Consumption
• Less Use of Anti-Scalants
• Less Cleaning Required
In a R/O process, feed water of high TDS is pressurized against a semi-permeable membrane. Part of the feed water permeates through this membrane and is of low TDS (total dissolves solids or salts) due to the rejection of salts by the membranes. The remaining part of the water is designated as reject and is of high TDS. Thus, RO operation results in concentration of salts in the reject stream. Limitations of a Conventional RO Process When designing an RO system, the natural saturation limits of various salts should not be exceeded, otherwise these salts would precipitate and scale the RO membranes. Silica is the most common process limiting constituent. At most commonly found conditions of 77F and pH between 6.5 to 7.5, the solubility of silica is 120 ppm. If silica in reject water exceeds more than 120 ppm under the above conditions, scaling of membranes would occur. Thus, if feed water has Silica of 50 ppm, maximum concentration possible under normal conditions of conventional RO would be reached within just 2 cycles of concentration. This would translate into a possible permeate recovery of approximately 50% from the feed water.
The HERO™ process (compared to a conventional RO system), is designed to increase the Silica solubility in the RO reject water to 1500 ppm and beyond. With the increased solubility, recoveries beyond 90% can be successfully achieved, under the conditions previously described. The following illustration is a graphical representation of increase in Silica Solubility factor with increasing pH of the water. As you can see, at a pH of 10 and above, the Silica solubility increases exponentially. This fact is extensively utilized in the design of HERO™ systems.
High Efficiency Reverse Osmosis (HERO System)
1. WAC Ion Exchange Treatment
Wastewater from the SAC ion exchange flows into the WAC ion exchange step of the HEROTM process to remove all remaining hardness. Because the WAC resin is in acid form, this step also lowers the pH of the water to 4.5, and converts carbonate and bicarbonate to carbon dioxide. This unit is regenerated using a dilute solution of hydrochloric acid.
2. Membrane Degasification
From the WAC ion exchange outlet, the wastewater goes through degasification to remove carbon dioxide. This inexpensive step removes the buffering capacity of the water, minimizing pH adjustment costs. Acid addition prior to the degasifiers may be necessary to lower the pH to 4.5 for complete carbon dioxide conversion.
3. pH Adjustment and Reverse Osmosis (RO)
The final step in the HERO system is adjustment to pH 10 and RO treatment. Operating the RO at high pH avoids bio-fouling and silica scaling, and enhances silt rejection. Treated wastewater is returned to the rinse water make-up system at a flow of about 15 gpm. Dissolved solids are discharged to the sewer.
The typical RO process problems solved by the HERO process can be seen in the following table:
Some of the features of the HERO process:
Benefits of HERO:
• High Recovery
• High Salt Rejection
• High Flux
• Reduced Fouling
• Lower Operating Costs
• Reduced Maintenance
HERO is Perfect For:
• High Silica Water
• Cooling Tower Blowdown
• Tertiary Treated Eﬄuent (Sewage) High TOC / Biologically Active Water
Besides having technical superiority over conventional reverse osmosis, the HERO process oﬀers signiﬁcant economic advantage.
• Since the HERO system allows increased water recovery, the cost per gallon for pure water production is reduced.
Minimal Oﬄine Cleaning
• Operating data shows a frequency requirement of once a year for HERO vs. twice to three times a year for conventional reverse osmosis. Substantially reduce the time your system is oﬀ-line for the time your system is oﬀ-line for membrane cleaning
No propriety chemicals
• The HERO technology runs on common chemical feed requirements allowing you to cut the cost of expensive cleaning chemicals typically required.
• Due to HERO’s high pH operating environment, it is in a “continuous cleaning mode” thus eliminating redundancies in capital equipment to compensate for cleaning downtime.